U.S. Pat. No. 4,955,442 illustrates a steering mechanism for converting the rotational movement of a steering wheel to linear actuation of one or the other of a pair of pressure regulating valves connected to the main hydraulic steering control. That mechanism includes an input shaft and a single actuating cam which are co-jointly rotatable away from a neutral position in either direction. A cam follower assembly is cooperatively associated with a substantially spiral shaped cam slot defined in the actuating cam and includes a cam following roller wheel received in a guide slot. The cam follower assembly also includes a bell crank supporting the roller wheel and indirectly connected to the pilot valves so that rotating the input shaft in opposite directions imparts controlled displacement of the pressure regulating valves through a separate spring in each control valve.
One of the problems encountered therewith is that the spring tolerances of the valves must be tightly controlled to provide equal control pressures at the control port of each valve relative to a steering input so that rotating the steering wheel equally in opposite directions from the straight ahead position results in the same degree of left or right steering. Moreover, the preload on the springs must be precisely matched to insure that the initial ramp-up control pressures for left and right turns are equal so that steering commences at the same degree of steering wheel rotation from the straight ahead position. The term ramp-up control pressure commonly refers to the minimum regulated control pressure setting of a pressure regulating or pressure control valve and is a function of the spring preload.
Thus, it would be desirable to have a pressure regulating valve for use in a steering system for providing substantially equal ramp-up control pressures and substantially equal rates of pressure rise of the regulated control pressures in both directions of operation.